The French patent application FR-A-2 448 824 describes a videography system in which the data is locked byte by byte with the aid of scrambling bytes delivered by a pseudo-random generator. This generator is made up of a set of trigger circuits and exclusive-OR gates. The performance of this type of generator is relatively mediocre when implanted in software. By way of indication, it needs about 6 seconds to descramble a televised page containing 1 kilo-byte.
The European patent EP-A-0 083 998 (corresponding to the American patent U.S. Pat. No. 4,543,559) describes a more high-performing and particularly faster pseudorandom generator without being more complex. Furthermore, the period of the pseudo-random sequence generated is increased up to about 2.3.10.sup.30 bytes. To this effect, the generator includes one initialization input and three sub-units each constituted by logic exclusive-OR gates, various registers and one adder. These sub-units define three recurrent sequences. A logic output circuit makes it possible to combine various bits appearing in these three sub-units. This logic circuit delivers 5 bits constituting the least significant 5 bits of one scrambling byte, the bits 6 and 7 being preset at 0 and the 8th being one parity bit calculated by an adder.
Although satisfactory in certain respects, this generator does have the drawback of only delivering 5 pseudo-random bits.